1 Field of the Invention
This invention relates to a machine for forming, pressing and drying of a thin paper web, which consists of a forming unit and at least one press roll press against the Yankee drying cylinder.
2. Description of the Prior Art
Modern Yankee machines for thin paper webs have the possibility of reaching speeds of about 2000 m/min when the web grammage is about 17 g/m.sup.2. The machines consist of a forming section, often a so-called twin wire former, a web pick-up arrangement which transfers the formed web to a press felt, and a suction press roll over which the felt with the web is turned so that the latter is pressed directly against the heated Yankee cylinder. Normally a second "hot" press follows which in most cases is a blind-drilled roll. The web is then transported further on the cylinder under hot air blowing hoods until it is finally removed from the cylinder by a doctor.
This type of machine has many devices and functions that approach critical limits at high machine speeds. Such a one is the pick-up from the forming wire to the press felt, especially when the felt is new. The felt must have a smooth surface, a certain surface fiber fineness, density, and water content in order to function. If large water quantities must be applied to the felt in order to attain a good pick-up function, then this creates problems at the suction hot press. If instead a high vacuum is used for the pick-up roll, then this may result in suction of web fibers into the press felt which will then be clogged.
The paper web is transported towards the Yankee cylinder underneath the felt, which is also critical and may be jeopardized by a low felt water content, uneven felt surface, or an impermeable as well as too open felt.
The transport of this web around the suction hot press is very delicate at high speeds due to the high centrifugal forces that are created. This is therefore carried out at a high vacuum with large amounts of air.
If the water content of the web or the felt is too high, or if the vacuum is too low, part of the web will be cast away from the roll, necessitating a lowering of the speed.
It is well known that transporting the web in a special way to the presses without pick-up function and direction reversing over the rolls has been carried out with the so-called Periformer machines. On these machines the web is formed between the forming wire and the cylinder surface. The disadvantage with that is that forming in this way results in cooling the cylinder and that a large part of the cylinder circumference is used for forming so that only a reduced part remains for the drying of the web.